Sorting single wall carbon nanotubes by electronic structure using gel chromatography

In the last decade due demand of last generation of high technology materials, there is a tremendous interest in nanotechnology [1]. Due to their marvellous material properties nanomaterials differ from the isolated atom and the bulk phase. Nanotechnology aims to production and improvement of smaller, cheaper, lighter, faster device with more functionality and less raw material and energy. Therefore, nanotechnology deals with materials having a size of 1100 nanometer (nm). The properties of the material changes as the size of the material decreases. When the nanomaterials are considered, surface behaviour of the material dominates the behaviour of the overall material. Nanomaterials their mechanical, electrical, and optical properties improve. Therefore, they can be implicated to many fields such as electronics, chemicals, sensors, and biotechnology. The identification of the structure of fullerenes in 1985 by Kroto and his friends was a breakthrough in nanotechnology [2]. It was followed by the discovery of multi walled carbon nanotubes (MWCNTs) in 1991 and single wall carbon nanotubes (SWCNTs) in 1993 by lijima [3, 4]. The unique one-dimensional (1-D) quantum confined properties of carbon nanotubes (SWCNTs) have sparked considerable interest in the scientific and technological community [5,6]. As-prepared SWCNTs, however, naturally contain two different species of tubes namely the semiconducting SWCNT (S-SWCNT) and metallic SWCNT (M-SWCNT). Different species of carbon nanotubes are used for different applications, SWCNTs have the potential to revolutionize numerous applications where nanosized metallic and/or semiconducting components are required along with high strength [7,8] large flexibility [9] and superb chemical stability [10,11]. In particular, metallic SWCNTs are highly suited for nanoscale circuits, [12] ultrathin, flexible and transparent conductors, [13] supercapacitors, [14] field emitters, [15] Actuators, [16] and nanosized electrochemical probes [17]. Semiconducting SWCNTs on the other hand, are applicable for nanoscale sensors, [18] transistors, [19] and photovoltaic devices [20]. Statistically, one-third of these structures are metallic and the other two-thirds are semiconducting. It is therefore imperative to separate the two species of nanotubes before integrating it in electronics. Several methods are described in the literature to sort semiconducting from metallic SWCNTs , including dielectrophoresis, [21] DNA-assisted separation, [22] selective polymer wrapping, [23], Gel Agarose chromatography [24-26] , Density Gradient Ultracentrifugation (DGU) [27] and amine extraction [28]. Post-synthesis separation of metallic (m-SWCNTs) and semiconducting (s-SWCNTs) single-wall carbon nanotubes (SWCNTs) remains a challenging process. Gel Agarose chromatography is emerging as an efficient and large scale separation technique. However, the full (100%) separation has not been achieved yet, mainly due to the lack of understanding of the underlying mechanism. Here, we study the PH effect on the SWCNTs separation via gel Agarose chromatography. Exploiting a gel Agarose micro-beads filtration technique, was achieved up to 70% m-SWCNTs and over 90% s-SWCNTs. Chromatography allows for separation of metallic from semiconducting through the use of a stationary phase such as Sephacryl-200 or Agarose gels and a mobile phase of 1 wt% SDS. the interaction between nanotubes and the gel is expected to be similar to that in the SDS system, where s-SWCNTs have higher affinity towards the gel compared with m-SWCNTs. Consequently, during the elution, s-SWCNTs adsorb onto the gel to a much greater extent, which results in a longer time to flow through the gel than m-SWCNTs. Metallic nanotubes elute from the column and result in a brown fraction, while semiconducting SWCNTs elute second and result in a purple-blue-green fraction. The dominant mechanism affording this electronic SWCNT type separation is still a significant research area. For gel chromatography, either ally dextran or Agarose gel are mostly used in previously reported literature. Novelty of this research relies on using both of these gels for increasing the efficiency of the separation. According to optical absorbance spectra of separated s-SWCNT, small s-SWCNTs are adsorbed to dextran gel beads whereas large s-SWCNTs are adsorbed to Agarose gel beads. Considering these properties, our group have developed a novel mixed method for separation of m-SWCNTs and s-SWCNTs by using dextran and Agarose gels respectively. By applying a SWCNTs/SDS dispersion to a column containing dextran, small s-SWCNTs observed to be adsorbed to the gel and collected by changing the eluent. This process was repeated for obtained unbundle solution until there was no any absorption to the gel. Subsequently, the operation continues with Agarose gel beads for absorbing the large s-SWCNTs repeatedly until the end of adsorption process. In the end, we separated nearly all of the small and large s-SWCNTs and m-SWCNTs that passed through the final column as unbundle solution. This method while being low-cost, can be done in large-scale with very high efficiency (without missing large amount of initial raw SWCNTs). Chapter 2 of this thesis will be about SWCNTs structure and properties, In Chapter 3 will been discussed about SWCNTs separation methods and Chapter 4 will be about SWCNT characterization methods. Chapter 5 presents the experimental studies performed at this work. It includes SWCNTs separation processes and characterization of separated SWCNTs as metallic and semiconducting. Chapter 6 offers the results of SWCNTs separation and characterization. It includes evaluation spectroscopy piks of separated SWCNTs in this study with the over 90% purified s and m-SWCNTs that exist in literature. The overall results and recommendations for this study are given in Chapter 7.